Sound record and reproducing system



Patented Feb. 2, 1943 SOUND RECORD .AND REPRODUCING SYSTEM Walter J. Albershem, Great Neck, N. Y., assignor to Western Electric Company, Incorporated, a corporation ofNew York Application October 4, 1941, lSerial No. 413,639

4 Claims.

This invention relates to recording and reproducing systems and particularly to systems of this kind in which the records are of the lateralcut type.

Since the grooves of lateral-cut records are not of uniform width, it is customary to reproduce them with a stylus which makes contact with only thebottom. portion of the groove. While this prevents the rapid destruction of the groove walls at the pinched sections, it leaves the stylus without positive guidance and permits a great deal of uncontrolled vibration which appears as distortion in the reproduction.

To reduce this type of distortion without producing abnormal wear at. the pinched portions of the groove it has already been suggested that the groove be V-shaped and that the record be reproduced with a spherically vtipped stylus mounted in a reproducerlof such design that the stylus is capable of vertical motion at twice the highest signal frequency in addition to being free to vibrate in the lateral sense. In such a reproducing system the stylus is guided by the side walls rather than the bottom of the groove and at the pinched portions the stylus will merely track higher in the groove with the result that the groove deformation is greatly reduced. This is not a complete solution of the problem, however, since the stylus tip, being spherical, makes only a point contact with each side wall and the wear on the groove is still considerable particularly for the softer record materials.

The object of this invention is to reduce further the record groove distortion resulting from the pinch effect.

According to the invention the groove walls and the reproducing stylus are so shaped that the stylus makes a line contact with each Wall of the groove and maintains a line contact as the stylus moves upwardly in a pinched groove. In order to maintain contact even when the groove is narrowed during modulation, the groove profile must be a curve which remains congruent with tself when the groove is pinched or compressed laterally. This requires that the groove have side walls which are convex in the region adjacent the recording surface and that the convex portions follow symmetrical exponential curves. The bottom of the groove may be of any convenient shape since it is not used for driving the stylus. The reproducing stylus has an exponentially curved contact portion and a tip which is ground off to provide clearance between it and the bottom of the groove.

Lateral type reproducers which are free to deect'in a vertical sense and therefore suitable for reproducing records according to this invention are well-known in the art as shown, for example, by such patents as 2,027,168 to Harrison and 2,055,187 to Vieth.

In the drawing Fig. 1 is a sectional view of a portion of a recording according to the invention; Figs. 2 and 3 show a recording stylus for cutting grooves according to the invention; Figs. 4 and 5 show a reproducing stylus tracking normal and pinched grooves respectively and Fig. 6 is a plan view of the fragment of the record shown in Fig. l.

In Figs. 1 and 6 groovev l is a typical unmodulated groove in the surface of the record material 2, the groove 3 is a modulated groove with a section taken at the point of maximum excursion from the unmodulated position and the groove 4 is another modulated groove with a section taken at a point in the groove where the recorder stylus is moving radially of the record. If the Width of the unmodulated groove l is mo, then the modulated groove 3 will also be of width :no in Fig. 1 since at the point where the section of this groove is taken the recorder stylus was momentarlly at rest.

At the point where the section of groove 4 is taken, however, the recorder stylus was moving radially of the record with the result that the groove 4 is pinched to a width :1:0 cos o where is the angle between the direction of the groove at this point and the tangent to the unmodulated groove as shown in Fig. 6. It will be understood that the amplitude of the groove undulations in Fig. 6 has been greatly exaggerated to show this pinch eiect more clearly.

The side walls of the grooves are similar exponential curves extending from the surface 5 of the record and converging toward the bottom of the groove to some such points as 6 and 'I where they intersect the bottom portion which may be circular or of any other convenient shape.

Y Such a groove may be cut by a stylus of the type shown in Figs. 2 and 3 in which the side view `of the stylus tip 8 is conventional but the cutting face 9 has an exponential portion I0, l0 above which it has straight sides ll I l with a QO-degree angle between them. Below the exponential region the cutting face is rounded off at the tip i2 to limit the depth of the groove.

The reproducing stylus L3 may be 'of circular cross section throughout but its sides have an exponential region It fitting the groove between the points i6 and Il over a lengthof about onehalf the groove depth as shown in Fig. 4. The

tip portion I is rounded ofi on a greater radius of curvature than the groove so that the stylus always rides on the side Walls of the groove.

When the groove becomes pinched as in Fig. 5 the stylus merely rides higher in the groove but due to the exponential curvature of the side Walls a, line contact is maintained and the groove undulations are reproduced without any substantial increase in pressure at the pinched sections.

The fact that an exponentially curved grooved wall meets the requirements for line contact with varying groove width will be more clearly understood from the following mathematical treatment of the problem:

In order to maintain a line contact the groove profile obviously must follow a curve which remains congruent with itself when the groove is pinched or laterally compressed. In other Words in a groove Where the width a: is a function of y. the groove depth, compression by the factor (a) should produce a vertical shift in the curve (c) and this means that If the groove width (x) is an exponential function of the groove depth y then where e is the base of Naperian logarithrns and Kr and K2 are arbitrary constants like a and c in Equation 1 above. From (2) :Kreml-*Mdm a) 4) If K2 is chosen so that K2=log. a

then from (4) a=k1ex2 1l+ =fw+cl (5) as required by Equation 1.

If y is measured downward from the record surface, a narrowing exponential groove with a surface width :ro is characterized by the equation:

and the wall slope of each side is:

The wall slope at the surface is and if this is to be i5-degree as specified above then l im g5 2 *E* 1 2 (9) and 2 li-zo Substituting this value for K in (6) above l2! wir =roe I (11) and this equation defines the particular exponential curvature required for the recording stylus described.

While an exponential curve is the only one which meets exactly the requirements for line contact, considerable improvement over conventional systems Will be obtained with any curve which is a reasonable approximationsince with most materials the groove walls Will yield slightly to conform with the contour of the reproducing stylus without suii'ering permanent deformation.

What is claimed is:

1. A sound record comprising a recording surface and an undulatory groove therein having convex side walls approaching each other along substantially exponential curves.

2. A sound record according to claim 1 in which the bottom of the groove is concave and each side wall has a slope of approximately 45- degrees at the record surface.

3. The method of recording and reproducing a lateral cut sound record Without substantial distortion due to the pinch eiect which comprises recording the sounds in a groove having side walls approaching each other over a portion of their lengths along substantially exponential curves and reproducing the record with a reproducer having a vibratory system free to deflect in a vertical as -well as a lateral sense and a stylus driven solely by the exponential portion of the groove.

4. In a sound reproducing system a sound record having a laterally undulating groove with convex side walls approaching each other along substantially exponential curves and a reproducer having a vibrating system free to vibrate in both lateral and vertical senses and a stylus with a tip contoured to rest against only the exponential portion of the groove walls.

WALTER J. ALBERSHElM. 

